a(n) = 2*a(n-1) + 3*a(n-2), with a(0) = 1, a(1) = 9.

Terms

a(0) =1a(1) =9a(2) =21a(3) =69a(4) =201a(5) =609a(6) =1821a(7) =5469a(8) =16401a(9) =49209a(10) =147621a(11) =442869a(12) =1328601a(13) =3985809a(14) =11957421a(15) =35872269a(16) =107616801a(17) =322850409a(18) =968551221a(19) =2905653669a(20) =8716961001a(21) =26150883009a(22) =78452649021a(23) =235357947069a(24) =706073841201